code: def test_selected_index(self):
      widget = self.widget(self.children, selected_index=1)
      state = widget.get_state()
      assert state["selected_index"] == 1
  
imports: from unittest import TestCase
  from traitlets import TraitError
  from ipywidgets.widgets import Accordion, Tab, Stack, HTML
label_desc: Use image node_id 4 for calling the TestStack obj's underlying member method code with example usage: obj.test_selected_index() without return types